The unusual combination of anisotropic and fluid behavior of liquid crystals has resulted in their use in electrooptical switching and display devices, where their electrical, magnetic, elastic and/or thermal properties can be used to cause changes in alignment. Optical effects can be achieved, for example, using birefringence, the inclusion of dye molecules which absorb dichroically ("guest-host mode") or light scattering.
There has recently been increasing interest in, in particular, ferroelectric liquid crystals as a display medium in electrooptical components (displays) (for example Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, October Meeting 1985, San Diego, Calif. USA).
Practical use of ferroelectric liquid crystals in electrooptical displays requires chiral, tilted, smectic phases, such as S.sub.c phases [R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. Physique 36, L-69 (1975)], which are stable over a broad temperature range. This aim can be achieved using compounds which themselves form such phases, for example S.sub.c * phases, or by doping compounds which form nonchiral, tilted, smectic phases with optically active compounds [M. Brunet, C. Williams, Ann. Phys. 3, 237, (1978)].
Furthermore, use of ferroelectric liquid-crystal mixtures in electrooptical components requires a uniform planar alignment of the liquid crystals in order to achieve high contrast. It has been shown that a uniform planar alignment in the S.sub.c phase can be achieved if the phase sequence of the liquid-crystal mixture with decreasing temperature is: isotropic.fwdarw.nematic.fwdarw.smectic A.fwdarw.smectic C (see, for example, K. Flatischler et al., Mol. Cryst. Liq. Cryst. 131, 21 (1985); T. Matsumoto et al., p. 468-470, Proc. of the 6th Int. Display Research Conf., Japan Display, 30 September-2 October 1986, Tokyo, Japan; M. Murakami et al., ibid., p. 344-347).
For ferroelectric (chiral smectic) liquid-crystal mixtures, the pitch of the helix must additionally be large, i.e. greater than 5 .mu.m, in the S.sub.c * phase and very large, i.e. greater than 10 .mu.m or infinite, in the N* phase.
The optical response time, .tau.[.mu.s] of ferroelectric liquid-crystal systems, which should be as short as possible, depends on the rotational viscosity of the system .gamma.y[mPas], the spontaneous polarization P.sub.s [nC/cm.sup.2 ] and the electrical field strength E[V/m] in accordance with the equation ##EQU1##
Since the field strength E is determined by the distance between the electrodes in the electrooptical component and by the applied voltage, the ferroelectric display medium must be of low viscosity and have high spontaneous polarization so that a short response time is achieved.
Finally, in addition to thermal, chemical and photochemical stability, a low optical anisotropy .DELTA.n, preferably &lt;0.13, and a low positive or preferably negative dielectric anisotropy .DELTA..epsilon. are required (see S. T. Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SID Symposium, Oct. Meeting 1985, San Diego, Calif. USA).
All these requirements can only be achieved together by means of mixtures comprising a plurality of components. The base (or matrix) used preferably comprises compounds which if possible already have the desired phase sequence I.fwdarw.N.fwdarw.S.sub.A .fwdarw.S.sub.c. Further components of the mixture are frequently added to reduce the melting point and to broaden the S.sub.c and usually also the N phase, to induce optical activity, for pitch compensation and for matching the optical and dielectric anisotropies, but the rotational viscosity, for example, should not be increased if possible. It has become apparent that the use of mixture components which contain only two aromatic rings results in a low viscosity of the mixture.
Some of these components individually and also certain mixtures have already been disclosed in the prior art. However, since development, in particular of ferroelectric liquid-crystal mixtures, can in no way be regarded as complete, the manufacturers of displays are interested in various mixtures, also because, inter alia, conclusions on the quality of the liquid-crystalline mixtures too can only be achieved through the interaction of the liquid-crystalline mixtures with the individual components of the display devices or the cells (for example the alignment layer).
It is known that certain derivatives of phenylpyrimidine, in particular 5-alkyl-2-(4-alkoxyphenyl)pyrimidines, are able to form S.sub.c, S.sub.A and N phases (see D. Demus and H. Zaschke, "Flussige Kristalle in Tabellen" [Liquid Crystals in Tables], VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig 1974, pp. 260-261) and can also be converted into ferroelectric liquid-crystal mixtures through addition of optically active dopes [see L. M. Blinov et al., Sow. Phys. Usp. 27 (7), 492 (1984); L. A. Beresnew et al., Ferroelectrics, 59 [321]/1 (1984), presented at the 5th Conference of Soc. Countries on Liquid Crystals, Odessa, USSR, October 1983; DE-A 35 15 347, EP-A 0 206 228 and EP-A 0 225 195].
It is also known that relatively low melting points and a broadening of the desired liquid-crystalline phases can be achieved by mixing a plurality of liquid-crystalline compounds [see D. Demus etal., Mol. Cryst. Liq. Cryst. 25, 215 (1974), J. W. Goodby, Ferroelectrics 49, 275 (1983)], and that the melting point depression is the more pronounced the more the mixture components also differ structurally from one another (for example J. S. Dave et al., J. Chem. Soc. 1955, 4305). This also applies to the melting point depression in systems which have the phase sequence X.revreaction.S.sub.c .revreaction.S.sub.A .revreaction.N.revreaction.I, which is ideal for the production of electrooptical components. In this case, however, other essential characteristic quantities tend to be retained only if the components of the mixture are structurally similar and themselves have this phase sequence. The two objects--melting point depression and shift in the lower temperature limit of the S.sub.c phase toward lower temperature on the one hand and substantial retention of their other characteristic quantities on the other hand--are thus contradictory.
It has already been stated that liquid crystals having a terminal cyclopropyl group (see DE-A 3 915 804 and DE-A 3 839 330) and liquid crystals having a geminal dimethyl substitution (see DE-A 4 003 012) or having a dimethylsilyl substitution (see DE-A 3 827 600) in the chain are suitable for use in liquid-crystalline mixtures.
However, the dimethyl-branched compounds have comparatively narrow hemtic phase ranges. This is particularly true for compounds having only two aromatic rings, which, due to their low viscosity, are preferred over compounds having three aromatic rings. However, the compounds having two aromatic rings which are presented in DE-A 4 003 012 and DE-A 3 827 600 either have no hemtic or no S.sub.A phase and/or relatively high melting points and unsatisfactory melting point properties in mixtures.
Also in the case of the substances having two aromatic rings which are presented in DE-A 3 915 804, the examples having the highest clearing points have no S.sub.A phase. Although the use of these components in mixtures frequently results in an increase in the temperature of the nematic and S.sub.c phases, it also results in the disappearance of the S.sub.A phase.